Actually this question is based on a Blog Entry, which discusses the topic for FF2. But how does this work with FireFox 3?
I know that there must be a workaround, because recently I visited a site saying: "Please disable AdBlock+ before entering this site" :(
There's no general-purpose way to do that, for security reasons. Even the FF2 version mentioned there was specific to that particular extension -- you can't assume identically-named and -placed icons for all extensions.
The AdBlock-spotters, as Stephen presumed, tend to be behavior-based and specific to that extension. Likewise Greasemonkey-spotters, etc.